1. Field of the Invention
The present invention relates to a failure detection apparatus for an internal combustion engine, and more particularly, to techniques enabling reliable detection of abnormality in an air flow sensor.
2. Description of the Related Art
Recently, in order to prevent harmful exhaust gas from being emitted from an engine mounted on a motor vehicle, various control means are used to improve exhaust gas characteristics. These control means operate based on information supplied from various sensors and the like, to improve the exhaust gas characteristics.
If any of the sensors etc. falls, however, the exhaust gas characteristics may possibly be deteriorated, and accordingly, there has been a demand for reliable detection of failure of such sensors etc. Recently, vehicles equipped with an on-board diagnostic system (OBD etc.) have been developed and put to practical use, with a view to further improving the exhaust gas characteristics.
Information from the various sensors etc., especially, information from an air flow sensor (AFS) is used for many purposes including the control of an after-treatment device, EGR (exhaust gas recirculation), etc., and failure of the air flow sensor greatly affects the exhaust gas characteristics. Accordingly, diagnosis of the air flow sensor is of especial importance.
In the diagnosis of the air flow sensor, however, failure of the sensor is usually determined by comparing a reference value set in advance based on operating conditions, such as engine speed, fuel injection quantity, engine torque, throttle opening (throttle valve opening), manifold air pressure, etc., with an output value from the air flow sensor. Thus, in cases where EGR gas is introduced into the intake system, the quantity of fresh air varies depending on the opening of an EGR valve, giving rise to a problem that failure of the air flow sensor cannot be determined with accuracy.
While the opening of the EGR valve varies, the diagnosis of the air flow sensor may be suspended. Such suspension of diagnosis is, however, not desirable because it leads to a substantial reduction in the period for diagnosing the air flow sensor.